This invention relates generally to an acoustic muffler, and more particularly to a muffler for use in conjunction with a motor vehicle internal combustion engine to reduce exhaust noise, although it also may be used in other applications of silencing a fluid flow.
A muffler is generally connected to the outlet of an internal combustion engine exhaust gas system to reduce exhaust noise from the engine. There are two general classifications of mufflers, reactive and dissipative. Reactive mufflers are usually composed of several chambers of different volumes and shapes connected together with pipes. Reactive mufflers tend to reflect the sound energy back to the source. Dissipative mufflers are usually composed of ducts or chambers which are filled with acoustic absorbing materials. These materials absorb the acoustic energy and transform it into thermal energy.
Reactive mufflers are most useful when the source noise is composed of pure tones at specific, fixed frequencies, and when the fluid to be muffled is a hot, dirty, high-speed gas flow. Reactive mufflers are particularly useful for low frequency applications and for those installations where high temperatures or flammable gasses restrict the use of dissipative materials. Reactive mufflers are often constructed of baffles, reverse flow passages, or multiple tubes. These configurations produce a relatively high pressure drop, causing a back pressure at the exhaust of an engine, thus restricting engine performance. Back pressure produced by passenger car mufflers can range as high as three to seven pounds per square inch at maximum engine power. As a result it is desirable to employ a "straight-through" configuration to effectively eliminate back pressure.
Dissipative mufflers are useful when the source produces noise in a broader frequency band. They are particularly effective at high frequencies, but precautions must be taken if the fluid flow has a high velocity or temperature or if it contains particles or is corrosive. The most simple dissipative muffler is constructed by lining the interior of a duct with sound absorbing material. At high velocity, facing materials such as wire screens or perforated metal sheets are necessary to prevent erosion of the sound absorbing material. Dissipative mufflers produce a relatively low pressure drop with high attenuation at predominately middle and high frequencies. A typical dissipative muffler may be effective above approximately 500 Hertz. The approximate attenuation per linear foot is a function of the acoustical absorption coefficient of the absorbing material, the volume of the duct, as well as the frequency of the sound. Mufflers are generally tuned to attenuate specified frequencies. This tuning is volume dependent. A longer muffler will attenuate lower effective frequencies. The maximum efficiency for a simple dissipative muffler occurs at a frequency at which the width of the duct is between one half and twice the wavelength of the sound.
Mufflers which consist of a combination of the reactive and dissipative types are known in the art in a variety of configurations.